The function that needs to be called in imsanity is this:
$params array needs to look like this (to match wordpress standard upload handler):
$params['type'] = image mime type (ex 'image/jpeg')
$params['file'] = path to original file
$params['url'] = public url to file
that function in imsanity will return the exact same $params array that was passed in, but possibly the file may be changed, so the calling function needs to check to see if the filename was changed (for example a .bmp might be changed to a .jpg after the resize
if any upload plugin can support that then it could use imsanity to resize images just with that one function call. or if the plugin has it's own hook, it could work as well as long as the plugin will check the return value to see if the filename has been changed.